1.1.3 · D5Matter, Measurement & the Mole
Question bank — Separation techniques — filtration, distillation, chromatography, centrifugation, sublimation
True or false — justify
Filtration can produce pure water from salt water.
False. Filtration separates only by particle size, and dissolved Na⁺/Cl⁻ ions (~0.003 μm) are thousands of times smaller than filter pores (~2–10 μm), so they pass straight through with the water.
In distillation, the liquid you collect (the distillate) is the one with the higher boiling point.
False. The lower-boiling component vaporizes first, travels to the condenser, and is collected as the distillate; the higher-boiling component stays behind in the flask.
Simple distillation can separate ethanol and water into two pure liquids.
False. Ethanol and water form an azeotrope at 95.6% ethanol, at which vapor and liquid have the same composition, so distillation cannot enrich past that point.
In paper chromatography, the component that travels farthest is the most polar.
False. The farthest-travelling spot is the least polar (or most soluble in the mobile phase); polar components cling to the polar stationary phase and lag behind.
Centrifugation changes the chemical identity of the components it separates.
False. Centrifugation is a physical method — it only uses density differences under high artificial gravity; no bonds break and no new substances form.
Sublimation-based separation requires the mixture to first be melted into a liquid.
False. Sublimation goes straight from solid to gas without any liquid stage; that direct transition is the whole point, letting a subliming solid escape a non-subliming one.
If two liquids have boiling points 3 °C apart, simple distillation is the right choice.
False. Boiling points that close give overlapping vapor compositions, so you need fractional distillation (many vaporization–condensation cycles in a column) to resolve them.
A larger value means the component spent more time stuck to the stationary phase.
False. = distance travelled by component ÷ distance travelled by solvent, so a larger means it moved more with the mobile phase and stuck to the stationary phase less.
Filtration and centrifugation both separate a solid from a liquid, so they are interchangeable.
False. Filtration needs the solid particles to be larger than the pores; centrifugation works even for tiny suspended particles that would slip through a filter, using density instead of size.
Spot the error
"I'll distill sugar water by heating until the sugar vaporizes and condenses in the receiver."
The error is assuming everything vaporizes: sugar caramelizes and decomposes near 186 °C before it could boil, so you collect pure water and are left with burnt sugar — distillation only works if a component vaporizes before it decomposes.
"Since salt dissolves in water, I can filter it out with fine enough filter paper."
The error is treating dissolved solute like a suspended particle; even the finest paper has pores far larger than an ion, so no filter removes a dissolved solute — you need distillation, evaporation, or reverse osmosis.
"My distillate came out 100% ethanol from wine after one pass."
The error ignores Raoult's law and the azeotrope: a single vaporization gives a mixture vapor (~60% ethanol), and ethanol–water caps at 95.6%, so a single simple distillation cannot reach pure ethanol.
"I ran chromatography with a pure nonpolar solvent on nonpolar paper and got great separation."
The error is a mismatch of principle: if both phases are nonpolar, polar and nonpolar components are treated too similarly (little differential sticking), so the spots barely separate — you need a polarity contrast between phases.
"The residue is the liquid that collects below the filter."
The names are swapped: the residue is the solid trapped on the paper, and the filtrate is the liquid collected below.
"Fractional distillation is just simple distillation done twice, so it always beats simple distillation."
The error is thinking more is always better: for boiling points far apart (>25 °C) simple distillation already gives clean separation, and the fractionating column only adds unnecessary time and heat loss.
"To separate iodine mixed with sand, I'll dissolve both in water and filter."
The error picks the wrong property: iodine is only slightly water-soluble and this wastes its key trait — iodine sublimes on gentle heating while sand does not, so sublimation is the direct route.
Why questions
Why does the lower-boiling component vaporize first in distillation?
Because at a given temperature its molecules have weaker intermolecular attractions, so its vapor pressure reaches atmospheric pressure at a lower temperature — that condition is exactly what "boiling" means.
Why do we cool the condenser with running water in distillation?
The rising vapor must lose its latent heat to turn back into liquid; the cold water jacket continuously carries that heat away so the vapor condenses reliably into the receiving flask.
Why does a polar molecule move slowly in chromatography on polar silica or paper?
"Like attracts like": a polar molecule forms strong dipole and hydrogen-bonding interactions with the polar stationary phase, so it spends more time held back and less time moving with the solvent.
Why can centrifugation separate blood into layers that ordinary settling cannot separate quickly?
Spinning creates an effective gravity thousands of times stronger than Earth's, so denser components (red cells) sink and lighter ones (plasma) rise within minutes instead of hours.
Why is distillation so energy-intensive for desalination?
You must supply the full latent heat of vaporization to boil the water (turning liquid to gas costs far more energy than just warming it), which is why it takes roughly 40–60 MJ per cubic metre.
Why does the mobile phase rise up the paper on its own in paper chromatography?
Capillary action — the attraction between the solvent and the paper fibres pulls the liquid upward through the narrow spaces without any pump.
Why do we choose sublimation instead of distillation to purify a solid like iodine or camphor?
These solids pass directly from solid to gas and back, avoiding a messy liquid stage, so subliming them straight onto a cold surface gives a clean solid while non-subliming impurities stay behind.
Edge cases
What happens if you filter a mixture where the "solid" is actually fully dissolved (a true solution)?
Nothing separates — a solution has no particles larger than the pores, so the entire mixture passes through unchanged as filtrate.
What if two liquids in a mixture have identical boiling points?
Neither simple nor fractional distillation can separate them, because there is no vapor-pressure difference to exploit; you must switch to a different property such as polarity (chromatography) or add another substance to break the tie.
What happens in distillation if a dissolved solute decomposes below its own boiling point?
The solvent still distills over cleanly, but the solute never vaporizes — it chars or decomposes in the flask, so you recover the solvent, not the solute.
Can you separate two components by centrifugation if they have exactly the same density?
No — centrifugation relies entirely on density differences, so identical densities give no layering no matter how fast you spin.
What if a chromatography component has zero affinity for the stationary phase?
It travels essentially with the solvent front, giving an close to 1, and it cannot be resolved from any other equally non-sticking component.
Is filtered coffee "pure water plus flavour," and could you filter the flavour back out?
No — the caffeine, oils, and flavour compounds are dissolved, so they pass through the filter with the water; filtering again would remove nothing because there are no oversized particles left.
What happens if the mixture in sublimation contains two solids that both sublime at similar temperatures?
They vaporize together and re-deposit together, so sublimation fails to separate them — you'd need a temperature-staged setup or a different property difference entirely.
Recall One-line rule for choosing a technique
Ask which single physical property the components differ in most, and pick the method built for it. Property → method ::: Size → filtration; boiling point → distillation; polarity/affinity → chromatography; density → centrifugation; solid-to-gas → sublimation.